Group III nitride semiconductors have a direct transition band gap with energy corresponding to that from the visible light region to the ultraviolet light region, and hence are capable of light emission with high efficiency. Therefore, commercialized products using the semiconductor light-emitting device such as a light-emitting diode (hereinafter referred to as LED) or a laser diode (hereinafter referred to as LD) have been developed. Furthermore, they have a potential of offering characteristics that conventional III-V compound semiconductors cannot offer.
In general, a group III nitride semiconductor is manufactured by the metal organic chemical vapor deposition method (hereinafter referred to as MOCVD method), with trimethyl gallium (hereinafter referred to as TMG), trimethyl aluminum (hereinafter referred to as TMA), trimethyl indium (hereinafter referred to as TMI), and ammonia (hereinafter referred to as NH3) as raw materials. The MOCVD method is a method of crystal growth in which a carrier gas including a gas of raw material(s) is carried to a surface of a substrate and the material(s) are caused to react on the heated surface of the substrate. Here, as the substrate, a monocrystalline wafer such as of SiC or Al2O3 (sapphire) (hereinafter referred to as hetero-substrate) is used. However, there is a large lattice mismatch between such a hetero-substrate and a group III nitride crystal that is epitaxially grown thereon. For example, there is a 16% lattice mismatch between GaN and Al2O3, and there is a 6% lattice mismatch between GaN and SiC. In the presence of such a large lattice mismatch, it is difficult to epitaxially grow a crystal directly on a substrate. Even if a crystal is grown, it is not possible to obtain a favorable crystal. Therefore, in the case where a group III nitride semiconductor crystal is epitaxially grown on a sapphire monocrystalline substrate or on an SiC monocrystalline substrate by the MOCVD method, methods as disclosed in Patent Document 1 and Patent Document 2 have been used where a layer that is called a low-temperature buffer layer made from aluminum nitride (hereinafter referred to as AlN) or AlGaN is deposited on the substrate, and a group III nitride semiconductor crystal is epitaxially grown on the low-temperature buffer layer at high temperatures.
In recent, substrates including a group III nitride have been commercially obtainable. However, they are still expensive and no advantage is found for their industrial use. On the other hand, as for wafers in which a group III nitride semiconductor is deposited on a topmost surface of a hetero-substrate by a technique such as the MOCVD method, it has been possible to obtain and manufacture them comparatively easily.
Studies in manufacturing a group III nitride semiconductor crystal by the sputtering method are underway as well. For example, Patent Document 3 describes deposition of GaN directly on a sapphire substrate by the sputtering method, with the aim of laminating high resistance GaN. The conditions used were: an ultimate vacuum of 5×10−7 to 5×10−8 Torr; argon and nitrogen gases circulated in the chamber; a gas pressure at the sputtering of 3×10−2 to 5×10−2 Torr; an RF voltage of 0.7 to 0.9 kV (20 to 40 W in terms of power); a distance between the substrate and the target of 20 to 50 mm; and a substrate temperature of 150 to 450° C. However, a foundation layer of the light-emitting device is not referred to as a target application of GaN. Moreover, no description of forming a layer on top of the GaN film is made.
Furthermore, it is disclosed in Non-Patent Document 1 that a GaN was deposited on a Si (100) surface and an Al2O3 (0001) surface by radio frequency magnetron sputtering using nitrogen gas. The substrate temperature was changed from room temperature to 900° C., with the deposition conditions of a total gas pressure of 2 mTorr and an input power of 100 W. According to the figure published in the research paper, the employed apparatus is one in which the target and the substrate are opposed to each other.
Furthermore, in Non-Patent Document 2, GaN was deposited with an apparatus in which the cathode and the target were opposed to each other and a mesh was interposed between the substrate and the target. According to this, the deposition conditions were: a pressure of 0.67 Pa in nitrogen gas; a substrate temperature of 84 to 600° C.; an input power of 150 W; and a distance of 80 mm between the substrate and the target.
Conventionally, attempts have been made to laminate group III nitride semiconductors such as GaN on a hetero-substrate by the sputtering method. However, with a substrate whose topmost surface is made from a group III nitride semiconductor, no case has been reported of a manufacture of a crystalline group III nitride semiconductor which is favorable enough to fabricate a device such as an LED with by the sputtering method.
To obtain a semiconductor film with a favorable crystallinity, it is necessary to rigidly define deposition conditions for the sputtering method. Furthermore, in a crystal deposition by a physical adsorption method such as the sputtering method, a growth mechanism is often adopted such that the crystallinity of the substrate is taken over without change. Consequently, if a crystal film is to be deposited by use of the sputtering method, the material and the crystallinity of the topmost surface of the substrate, on which a deposition is conducted, is important.
Patent Document 1: Japanese Patent No. 3026087
Patent Document 2: Japanese Unexamined Patent Publication, First Publication No. H04-297023
Patent Document 3: Japanese Unexamined Patent Publication, First Publication No. S60-39819
Non-Patent Document 1: Proceedings of the 21 st Century Consortium Symposium, Vol. 2, p. 295 (2003)
Non-Patent Document 2: Vacuum, Vol. 66, p. 233 (2002)